Engraving foil with a light-absorbing layer for electronic engraving



Dec. 10, 1968 E. HELLMIG EI'AL 3,415,698

ENGRAVING FOIL WITH A LIGHT-ABSORBING LAYER FOR ELECTRONIC ENGRAVING Filed March 9, 1965 INVENTORS; EPHAPD HELLM/G, WALTER KLE/ST.

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United States Patent 3,415,698 ENGRAVING FOIL WITH A LIGHT-ABSORBING LAYER FOR ELECTRONIC ENGRAVING Ehrhard Hellmig and Walter Kleist, Leverkusen, Germany, assignors to Agfa Aktiengesellschaft, Leverkusen, Germany, a corporation of Germany Filed Mar. 9, 1965, Ser. No. 438,231 Claims priority, application Germany, Mar. 20, 1964, A 45,550 9 Claims. (Cl. 156-1) ABSTRACT OF THE DISCLOSURE Foil for engraving of correctable half-tone images, has transparent support carrying layer of water-pervious binder in which is uniformly dispersed light-absorbing particles whose light absorption can be reduced by chemical action of aqueous bath. Binder can be plasticized and layer can be protected by covering water-impervious layer. After engraving dots, image is corrected by the chemical action which reduces optical area of dots.

This invention relates to an engraving foil for electronic engraving and to a process of making intermediate copies of means of the said engarving foil.

In the so-called electronic engraving for the production of printing plates a printing plate is cut out dot by dot from a metal or plastic plate or foil by means of a stylus; the depth of penetration of the stylus into the material being engraved, and thus the size of the dots on the printing plate, is controlled electronically from the original which is to be reproduced.

It is also known that the engraved plate can either be used directly as a printing plate, e.g., for relief printing, or as an intermediate copy for the production of a printing plate. In the latter case, the material being engraved must be transparent but the raised portions of the engraved plate or foil must absorb the copying light. The engraving material consists of a transparent and usually colorless support, for instance a foil of a polymer with a light-absorbing layer coated thereon, for example, of carbon black in gelatin. Suspensions of metal powders (e.g. zinc or aluminum) in layers of nitrocellulose have also been used for the same purposes. Engraving foils with a colored outer layer or a layer which absorbs ultraviolet radiation are known.

However, all of these foils have the drawback that the tone values of the processed engraved foil can no longer be changed; the forms and sizes of the indentations remain the same in the copy as they were engraved. by the engraving machine. In many cases it is, however, highly desirable, to correct the engraved intermediate copy after the engraving step in order to improve certain defects in the tone values or other defects.

It is an object of the invention to provide an engraving foil for electronic engraving, the photographic properties of which can be corrected. A further object is to provide a process for the production of intermediate copies.

The above objects have been attained by using an engra-ving foil which consists of a transparent support and a swellable layer containing a soluble or bleachable light-absorbing substance. According to a preferred embodiment the engraving foil additionally contains a transparent protective layer on the said light-absorbing layer.

The material according to the invention is shown diagrammatically in the accompanying drawings, in which FIGURE 1 is a cross-sectional view of the engraving foil pursuant to the present invention, before it is engraved.

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FIGURE 2 is a similar view of the foil after engraving and correcting.

FIGURE 3 is a plan view of a portion of the foil of FIGURE 2 Arranged on a transparent support 1 is a layer 2 which contains one or more light-absorbing substances, e.g. finely divided pigments or dissolved dyes, in a swellable binding agent, it being possible for the said substances to be dissolved or at least decolorized by suitable baths. Arranged above this layer is a transparent covering 3.

The individual layers and the support can be of the following composition:

(1) SUPPORT Any transparent film-forming polymer can be used, preferably those which are suitable as supports for photographic films, such as foils of polycarbonates in particular of bis-phenylol alkanes, polyesters preferably polyethylene terephthalate, cellulose esters such as cellulose acetate or cellulose acetobutyrate, polystyrene or polyvinylchloride in particular copolymers of poly-vinylchloride which contain a plasticizing amount, for instance 10-20 mol percent of further olefinically unsaturated monomers in polymerized form in the polyvinylchloride chain. Suitable monomers are vinylacetate, vinylidene chloride, maleic acid esters, fumaric acid esters or esters of acrylic or methacrylic acid, in particular with lower aliphatic alcohols having up to 5 carbon atoms. The polymers should have a high dimensional stability.

The thickness of the support depends on the requirements of the particular reproduction or printing process in which the engraving foil is to be used, for instance as a printing plate for relief printing, and on the required dimensional stability and rigidity. We have found that generally a thickness within the range of from about A to 2 mm. are quite adequate to obtain the desired results. If the foil according to the invention is used as an intermediate copy, a thickness of between about 0.2 mm. and 1 mm. is suitable. In some cases it may be advantageous to use a matted support or to apply a separated matted layer on the back or between the light absorbing layer and support. The support may, moreover be provided with a separate antistatic layer or may also contain substances having an antistatic action, which prevent or at least reduce the electrical charging of the foil and thus the dust-attracting action thereof.

(2) LIGHT-ABSORBING LAYER The light-absorbing layer advantageously is applied to the support by means of a subbing layer, to improve the adherence between layers.

The light-absorbing layer has a thickness of about 3 to 20 microns preferably about 5 to 15 microns.

As light-absorbing compounds can be used organic or inorganic dyes, which can be removed from the layer without destroying the layer itself, can be used. Suitable are finely divided metals, preferably finely divided silver such as that used as antihalation layer in color photographic materials or black-and-white reversal materials. The finely divided silver can be bleached in the so-called Farmers reducer, described below. Suitable solvents for metals, are for instance, acids. Another suitable inorganic compound is colloidal manganese dioxide or pyrolusite, which is likewise used for antihalation layers in photographic films and which can be decolorized by a bisulfite solution.

Among the organic dyes there is to be mentioned the large group of the so-called antihalo dyes, which are known to be capable of being dissolved or decolorized in water or aqueous alkalis or acids.

The light-absorbing layer may also contain dyes which are known from the silver azo dye-bleaching process and which are decolorized in the presence of small quantities of silver as catalysts, by hydrogen peroxide, thiourea, acid potassium bromide or potassium iodide solutions (see, for example, Eder, Rezepte and Tabellen, Knapp, Halle, 1948, pp. 168 and 245).

The light-absorbing layer advantageously is black but generally the layer need only to be capable of absorbing the actinic light which is to be used in the subsequent printing process. It is obvious that the color of the lightabsorbing layer depends on the spectral sensitivity of the printing material onto which the engraved image is to be copied. For example, if the printing material is only blue-sensitive it is suflicient if the light-absorbing layer is yellow; with orthochromatic sensitivity a red color of the light-absorbing layer is sufficient; if it is substantially sensitive to ultra-violet, the layer must contain substances which absorb ultra-violet light. Nevertheless, in all cases a light-absorbing substance which is black or at least very dark in color is preferred, because it permits the engraved image to be most reliably checked by sight.

As binding agents for the light-absorbing layer may be employed a hydrophilic water-permeable colloidal material such as gelatin or any other colloidal albumin, a cellulose derivative or a synthetic resin, for instance, a polyvinyl compound. Colloids which may be used ar polyvinyl alcohol or a partially hydrolized polyvinyl acetate; a highly hydrolyzed cellulose ester such as cellulose acetate hydrolyzed to an acetyl content of about 20 to 25%, a polyacrylamide, vinyl alcohol copolymers with vinyl chloride, carboxyethyl cellulose, alginic acid or derivatives thereof, such as salts or esters, or polyvinyl pyrrolidone.

According to a preferred embodiment of the invention the light-absorbing layer is plasticized by the addition of suitable plasticizers. Plasticized layers can be engraved more easily. Without these additives, the layers are often too brittle for the engraving, especially if the layers consist of gelatin and if the layer has been exposed for a relatively long time to dry climatic conditions. 'In these circu-mstances, the light-absorbing layer can be stripped from the support during the engraving operation, or smaller screen dots can break-01f.

For this reason ordinary photographic silver halide layers which have been blackened by uniform exposure and subsequent development are of limited utility for electronic engraving.

The chemical structure of the plasticizer is not critical and can be selected in accordance with the binding agent which is tobg used for the light-absorbing layer.

Suitable as plasticising additives are, for example, hydrosols, and in particular for gelatin layers copolymers of esters of acrylic acid or methacrylic acid in particular esters with lower aliphatic alcohols having up to 5 carbon atoms, with acrylonitrile, acrylamide, styrene or butadien copolymerisates. Graft polymers of gelatin derivatives with esters of acrylic acids preferably with lower aliphatic alcohols having up to 5 carbon atoms and other polymerisable compounds are also suitable. Other known plasticisers are glycerine or other polyhydric alcohols, fatty acid amides, cyclic amides such as caprolactam and the acylation products thereof and also polyethylene glycol and derivatives thereof, such as esters, ethers and condensation products with dibasic carboxylic acids, phosphoric acid esters of polyethylene glycol or of pentaerythritol or mixed esters thereof, and also ethers of pentaerythritol, more especially cyanethyl ether. Suitable products are, for example, described in British Patent Nos. 922,251 or 926,689 or German Patents Nos. 1,185,811 or 1,179,109.

Advantageously suitable are copolymers which contain 50-90% of an ester of acrylic or methacrylic acid with an alcohol of 1-5 carbon atoms, l-30% of acrylonitrile and 1-5% of acrylic or methacrylic acid, as described in British Patent No. 900,195.

The plasticizers are added to the layer in quantities 4 from l30%, based on the dry weight of the binding agent preferably gelatin.

The effect of these additives is to make the layer sufficiently pliable, so that satisfactory cutting is possible during the engraving without splintering the gelatin layer or causing any stripping from the support. In addition, the layer remains sufficiently resistant to the cutting operation and also to other mechanical effects, such as those which occur with the normal handling of the material, more especially during the operation of etching or bleaching with a brush or cottonwool pad.

(3) PROTECTIVE LAYER The protective layer which can have a thickness of 0.1 to 5 microns preferably 0.5-3 microns should consist of a transparent preferably water-impermeable or even water-repellent binding agent.

Suitable binding agents for the protective layer are for instance, cellulose ester of inorganic or organic acids, such as cellulose nitrate or cellulose acetate, polystyrene, or copolymerisates of styrene with other polymerisable vinyl monomers, or polystyrene, or hardened gelatin. The protective layer can be applied onto the light-absorbing layer from a solution or a dispersion of the binding agents.

If the light-absorbing layer consists of a binding agent capable of being hardened, the separate protective layer can be replaced with an additional superficial hardening of the light-absorbing layer. In the case of gelatin as binding agent for the light-absorbing layer this can be performed, for instance, by treating the said layer with an aqueous solution of a hardening agent, such as solutions of chrome alum, chromyl acetate or chromyl stearate or polymeric hardening agents such as poly acrolein b-isulfite, dialdehyde starch or the like.

The operation which takes place with the bleaching of an electronically engraved screen is illustrated in FIG- URE 2.

In that figure an electronically engraved screen dot is shown on a greatly enlarged scale in cross-section as in FIGURE 1, 1 is the support, 2 is the light-absorbing layer, 3 is the protective layer and F are the flanks of the screen dot (cut surfaces). After the cutting operation the screen dot has a dimension extending from A to B as seen in FIGURE 3. On treatment with Farmers reducer, the bleaching action of that reducer is advantageously effective in the direction of the arrow, while its action from above is greatly impeded if not entirely prevented, because of the layer 3. In the course of the bleaching operation, silver is removed so that the screen dot, becomes smaller. (A B in FIGURE 3.) It is surprising in this respect that, despite the penetration of Farmers reducer from the outer side, a strong reduction of the size of the dot is possible without the density of the dot being considerably reduced. With the thickness of the layer 2 about 6 microns, experience shows that a reduction of the dot diameter by 0.2 mm.=200 microns is possible before there is a considerable loss of density. It is apparent that the protective layer 3 renders possible a bleaching of the flanks by far beyond the factor 70, based on the thickness of the screen dot.

EXAMPLE 1 A transparent support with a thickness of 0.2 mm. and consisting of a polycarbonate based on 4,4'-bis-(hydroxyphenyl)-alkanes, which layer is provided with a subbing layer described in British Patent No. 808,629, has applied thereto a l-ight-absorbing layer with a thickness of 6 microns comprising finely divided silver in gelatin. This latter layer also contains hardeners and wetting agents. In order to improve the engraving properties of the layer, there is also added to the casting solution 65 ml. of a 32% aqueous suspension of a copolymer of of butyl acrylate, 9% of methyl methacrylate, 10% of acrylonitrile and 1% of methacrylic acid, based on 1 kg. of silvergelatin dispersion with a gelatin-water ratio of 0.07 and a silver-gelatin ratio of 0.32. The silver coating is 1.20 g. per square meter. Above this layer, there is applied a gelatin layer which is 2.5 microns thick and which is also provided with hardeners and wetting agents. The other side of the support is provided with an ordinary antistatic layer which is 1 micron thick.

An image is engraved, into this foil by means of a photoelectric engraving machine such as marketed under the name Varioklischograph by Messr. Dr. Hell, Kiel, Germany. Similar machines are marketed by the firm Fairchild Camera and Instrument Corporation. In general those machines comprises a pair of synchronously rotatable cylinders for individually supporting an engraving foil and an original image sheet and a pair of reciprocable carriages individually including a photoelectric pick-up system and a tool for cutting the engraving foil for respectively scanning the image-sheet cylinder and the engraving foil cylinder. The machine also includes means responsive to the output of the pick-up system for actuating the tool, means for reciprocating the carriages synchronously; and an electrical screen generator driven synchronously with the cylinders and connected to modulate the output of the pick-up system.

Tone value corrections are carried out on the picture after treatment with the Farmers reducer having the following composition:

Solution A G. Sodium thiosulfate 150 Thiourea 12 Made up with water to 1 liter.

Solution B Potassium ferricyanide 50 Made up with water to 1 liter.

Before use, 1 part of A and 1 part of B are mixed with 1 part of water.

The corrected, rinsed and dried screen image serves directly as intermediate copy for the exposure of an offset plate.

EXAMPLE 2 A support of a copolymerisate of 90 parts by weight of vinyl chloride and parts by weight of vinyl acetate is treated for 8 minutes at 85 C. in the following solution to improve the adherence between the support and the light absorbing layer:

N21 Cr O 2H O g Water cm. 120 Concentrated H 80 cm. 1000 After carefully washing and drying the support, a subbing layer is applied onto the treated surface of the support. The subbing layer which contains 0.2 g. of gelatin per square meter has the following composition:

The subbing layer is dried at about 40 C. and overcoated with the light-absorbing layer containing finely distributed silver in gelatin as binding agent. Per square meter there were applied 250 g. of an aqueous dispersion containing per 1000 ml.

water g. of gelatin 12 g. of colloidal silver and the common coating aids such as saponin and hardeners such as formaldehydes.

Thereafter the layer is dried. The processing of the above foil is accomplished as described in Example 1.

We claim:

1. An engraving foil for the production of correctable half-tone screened images, comprising (a) a transparent photographic film support carrying (b) a light-absorbing layer of a water-permeable binding agent containing uniformly distributed light-absorbing material that is removable without destroying the layer, and

(c) on said light-absorbing layer, a protective layer between 0.1 and 5 microns thick of a transparent water-impermeable binding agent.

2. A process for the production of tone corrected halftone-screened images including the steps of (1) making a relief image in an engraving foil comprising (a) a transparent photographic film support carrying (b) a light-absorbing layer consisting of a lightabsonbing material whose light-absorbing properties are reducible without destroying the layer, uniformly distributed in a water-permeable binding agent by chipping out portions of the light-absorbing layer to a depth slightly greater than the thickness of the light-absorbing layer by means of photoelectric engraving in a regular screen pattern of discrete separate indentations, the area of the indentations in each elemental image area being representative of the tone value of such elemental area in the original image,

(2) treating the raised portions of the engraved relief image the tone values of which are to be corrected with a bath that reduces the light-absorption areas of the light-absorbing layer of the raised portions, and

(3) washing and drying the corrected engraved relief image.

3. A process as defined in claim 2, wherein the lightabsorbing layer contains as light-absorbing material a finely distributed metal.

4. A process as defined in claim 3, wherein the lightabsorbing layer contains finely distributed silver as lightabsorbing material.

5. A process as defined in claim 3, wherein the binding agent of the light-absorbing layer essentially consists of plasticized gelatin.

6. A process as defined in claim 2, "wherein the engraving foil additionally contains an outer transparent protective layer coated onto the light-absorbing layer, and the said protective layer has a thickness of between 0.1 and 5 microns and consists of a water-impermeable binding agent.

7. An engraving foil for producing correctable halfton'e screened images comprising (A) a transparent film-forming support carrying (B) a light absorbing layer containing a light absorbing substance whose light absorbing properties can be reduced without destroying the layer, uniformly distributed in a hydrophilic water-permeable colloidal binding agent, and

(C) on the light absorbing layer a transparent, waterimpermeable protective layer.

8. The engraving foil according to claim 7 wherein the transparent film-forming support has a thickness between 0.1 and 2 mm. and is composed of polycarbonate, polyester, cellulose ester, polystyrene or polyvinyl chloride, the light absorbing layer has a thickness between 3 and 20 microns, the light absorbing material is finely divided metal, colloidal manganese dioxide or an organic dye, the hydrophilic water-permeable binding agent is gelatin, colloidal albumin, a cellulose derivative or a polyvinyl compound, said light absorbing layer containing from 1 to 30 percent by weight based on the binding 7 agent of a plasticizer and the protective layer has a thickness between 0.1 and microns and is composed of cellulose ester, polystyrene, copolymers of styrene and vinyl monomers or hardened gelatin.

9. A process of producing tone corrected halftone 5 screened images according to claim 6 wherein in the engraving foil the transparent film-forming support has a thickness between 0.1 and 2 mm. and is composed of polycanbonate, polyester, cellulose ester, polystyrene or polyvinyl chloride, the light absorbing layer has a thickness between 3 and 20 microns, the light absorbing material is finely divided metal, colloidal manganese dioxide or an organic dye, the hydrophilic water-permeable binding agent is gelatin, colloidal albumin, a cellulose deriva- References Cited UNITED STATES PATENTS Re. 23,914 12/1954 Boyajean 1786.6 1,848,782 3/1932 Hausleiter 96-45 XR 2,647,835 8/1953 Weaver 96--84 2,788,255 4/1957 Fanber 1786.6 2,827,726 3/1958 Stradar 10140'1.1 3,018,178 1/1962 Harrirnan 96-84 3,205,806 9/1965 Ahrens et a1. 101401.1 3,218,971 11/1965 Rowland 101-401.1

NORMAN G. TORCHIN, Primary Examiner.

tive or a polyvinyl compound, said light absorbing layer 15 R, H, SMITH, A i t Examiner,

containing from 1 to percent by weight based on the binding agent of a plasticizer and the protective layer has a thickness between 0.1 and 5 microns and is compose-d of cellulose ester, polystyrene, copolymers of styrene and vinyl monomers or hardened gelatin.

U.S.Cl.X.R.

9638, l01368, 395, 401.1; l567, 14; l786.6 

